Canon 50mm 1.2L HELP!!!

[shesees]

I was SUPER excited to upgrade in the 50mm department.  I got the Canon 50mm 1.2 L series just last week.  I ordered it brand new from Adorama.  I put it on my 5D Mark iii and tested it around the yard (with my chickens).  LOVED the quality!  I brought it out in the field with me to shoot an intimate wedding ceremony (20 guests).  It worked fine for the first 1200 or so shots.  Now every 5-6th image looks highly overexposed.  But the settings are exactly the same as the other images!  NO flash being used.  I haven't EVER experienced this with any of my other lenses (I have a lot).  Any advice?

ISO 1600  F2.8  1/320sec

[Stephen]

For what it's worth, when troubleshooting over the phone, we'd always ask you to test a "faulty" lens on another body, if you have one available, just to rule out the possibility that your body might be sending incorrect information to the lens, or that it's a wonky setting. 

We'd also, usually prefer for you to send the "faulty" lens along with your camera body so that we can reproduce the situation not only on our precision calibrated equipment, but also on your equipment - again, to rule out every situation. This will often prevent a lot of shipping back and forth of equipment, and take much less troubleshooting time - I used to telll people we like to test your equipment with your equipment, then we test your equipment with our equipment, and see where the faults lie, then we take it from there. 

Far more often than not, if just a lens is sent, we put it on our precision testing equipment, and nothing seems to be wrong. We double-check it, verify everything, and then send it back. When the problem pops back up, we have to have everything sent back in and start from square one.